Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped from a surface mud pit, through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool.
Fluids, such as oil, gas and water, are commonly recovered from subterranean formations below the earth's surface. Drilling rigs at the surface are often used to bore long, slender wellbores into the earth's crust to the location of the subsurface fluid deposits to establish fluid communication with the surface through the drilled wellbore. The location of subsurface fluid deposits may not be located directly (vertically downward) below the drilling rig surface location. A wellbore that defines a path, which deviates from vertical to some laterally displaced location, is called a directional wellbore. Downhole drilling equipment may be used to directionally steer the wellbore to known or suspected fluid deposits using directional drilling techniques to laterally displace the borehole and create a directional wellbore.
The path of a wellbore, or its “trajectory,” may be determined by collecting a series of direction and inclination (“D&I”) measurement at various points along the wellbore and by using known calculation methods. “Position,” as the term is used herein, refers position of the wellbore, referenced to some vertical and/or horizontal datum (usually the well-head position and elevation reference). The position may also be obtained using inertial measurement techniques. “Azimuth” may be considered, for present disclosure, to be the directional angular heading, relative to a reference direction, such as North, at the position of measurement. “Inclination” may be considered, also for present disclosure, to be the angular deviation of the borehole from the vertical, usually with reference to the direction of gravity. “Measured depth” may be considered, also for present disclosure, to be the distance measured along the wellbore from the surface location. Measured depth may include the driller's depth, and it may also include depth correction algorithms, that account for the elastic stretching and compression of the drill string along its length.
Directional wellbores are drilled through earth formations along a selected trajectory. Many factors may combine to unpredictably influence the trajectory of a wellbore. It is desirable to accurately measure the wellbore trajectory in order to guide the wellbore to its geological and/or positional target. Thus, it is desirable to measure the inclination, azimuth and depth of the wellbore during wellbore operations to estimate whether the selected trajectory is being maintained.
The drilled trajectory of a wellbore is estimated by the use of a wellbore or directional survey. A wellbore survey is made up of a collection or “set” of survey-stations. A survey station is generated by taking measurements used for estimation of the position and/or wellbore orientation at a single position in the wellbore. The act of performing these measurements and generating the survey data is termed “surveying the wellbore.”
Many factors may combine to unpredictably influence the trajectory of a drilled borehole. It is important to accurately determine the borehole trajectory in order to determine the position of the borehole at any given point of interest and to guide the borehole to its geological objective. Surveying of a borehole using existing methods involves the intermittent measurement of the earth's magnetic and gravitational fields to determine the azimuth and inclination of the borehole at the BHA under static conditions; that is, while the BHA is stationary. These “static” surveys are generally performed at discrete survey “stations” along the borehole when drilling operations are suspended to make up additional joints or stands of drillpipe into the drillstring. Consequently, the along hole depth or borehole distance between discrete survey stations is generally from 30 to 90 feet corresponding to the length of joints or stands of drillpipe added at the surface.
Surveying of wellbores is commonly performed using downhole survey instruments. Such instruments typically contain sets of orthogonal accelerometers, magnetometers and/or gyroscopes. Survey instruments are used to measure the direction and magnitude of the local gravitational, magnetic field and/or earth spin rate vectors respectively, herein referred to as “earth's vectors.” Various measurements correspond to the instrument position and orientation in the wellbore, with respect to earth vectors. Wellbore position, inclination and/or azimuth may be estimated from the instrument's measurements.
One or more survey stations may be generated using “discrete” or “continuous” measurement modes. Generally, discrete or “static” wellbore surveys are performed by creating survey stations along the wellbore when drilling is stopped or interrupted to add additional joints or stands of drillpipe to the drillstring at the surface. Continuous wellbore surveys relate to many measurements of the earth's vectors and/or angular velocity of a downhole tool obtained for each wellbore segment using the survey instruments. Successive measurements of these vectors during drilling operations may be separated by only fractions of a meter and, in light of the relatively slow rate of change of the vectors in drilling a wellbore, these measurements are considered continuous for all practical analyses. The art of continuous surveys is very well described in U.S. Pat. No. 6,633,816, which is assigned to the assignee of the present application and is incorporated herein by reference in its entirety.
Known survey techniques as used herein encompass the utilization of a variety of methodologies to estimate wellbore position, such as using sensors, magnetometers, accelerometers, gyroscopes, measurements of drill pipe length or wireline depth, Measurement While Drilling (“MWD”) tools, Logging While Drilling (“LWD”) tools, wireline tools, and the like.
Existing wellbore survey computation techniques use various methodologies, including the Tangential method, Balanced Tangential method, Average Angle method, Mercury method, Differential Equation method, cylindrical Radius of Curvature method and the Minimum Radius of Curvature method, to model the trajectory of the wellbore segments between survey stations.
Directional surveys may also be performed using wireline tools. Wireline tools are provided with one or more survey probes suspended by a cable and raised and lowered into and out of a wellbore. In such a system, the survey stations are generated in any of the previously mentioned surveying modes to create the survey. Sometimes wireline tools are used to survey wellbores after a drilling tool has drilled a wellbore and an MWD and/or LWD survey has been previously performed. In some examples, a wireline survey may be made of a partially drilled wellbore, and the results may be used in calculating the position of the wellbore once drilling commences again.
Uncertainty in the survey results from measurement uncertainty, as well as environmental factors. Measurement uncertainty may exist in any of the known survey methodologies. For example, magnetic measuring techniques suffer from the inherent uncertainty in global magnetic models used to estimate declination at a specific site. Similarly, gravitational measuring techniques suffer from movement of the downhole tool and uncertainty in the accelerometers. Gyroscopic measuring techniques, for example, suffer from drift uncertainty. Depth measurements are prone to uncertainty including mechanical stretch from gravitational forces and thermal expansion and compression from the weight on bit, for example.
Additionally, for each methodology, there is a trade-off between time required to complete the survey and the resulting resolution and degree of accuracy.
Various considerations have brought about an ever-increasing need for more precise wellbore surveying techniques. More accurate survey information is necessary to ensure the avoidance of well collisions and the successful penetration of geological targets.